Tales from the Transit of Venus

Sad Old Sun

Today is the transit of Venus, which, aside from being a totally rad
astronomical event, is also the perfect excuse to tell my favorite story
of an unlucky Frenchman (I have many). This is by no means new and, if
you’ve ever taken an astronomy course, you’ve probably already heard it.
It is perhaps the closest thing Astronomy has to a ghost story, told
though the glow of a flashlight on moonless nights to scare the
children. This is the story of Guillaume Le Gentil, a dude that just
couldn’t catch a break.

Guillaume Joseph Hyacinthe Jean-Baptiste Le Gentil de la Galaisière was
a Frenchman with an incredibly long name. He was also an astronomer,
though he hadn’t started out that way. Monsieur Le Gentil (as his
friends called him and so, then, shall we) had originally intended to
enter the priesthood. However, he soon began sneaking away to hear
astronomy lectures and quickly switched from studies of Heaven to those
heavens more readily observed in a telescope. Le Gentil happened to get
into the astronomy game at a very exciting time. The next pair of Venus
transits was imminent and astronomers were giddy with anticipation.
Though the previous transit of 1639 had been predicted, it was met with
little fanfare and only a few measurements. But the transits of 1761 and
1769 would be different. People would be ready. And the stakes were
higher this time, too. Soon after the 1639 transit, Edmund Halley (he of
the-only-comet-people-can-name fame) calculated that with enough
simultaneous measurements, the distance from the Earth to the Sun (the
so-called astronomical unit, or AU) could be measured fairly accurately.
Since almost all other astronomical distances were known in terms of the
AU, knowing its precise value would essentially set the scale for the
cosmos. Brand new telescopes in hand, the astronomers of Europe set sail
for locations all over the world.

Le Gentil had been assigned to observe the transit from Pondicherry, a
French holding on the eastern side of India. On March 26th, 1760, he
began his long sea voyage around the Cape of Good Hope towards India.

The voyage from France to India was a bit too long for the ship Le
Gentil hitched a ride on and he only made it as far as Mauritius (a
small island off Madagascar). Dropped off with all his equipment, Le
Gentil was left waiting for any ship at all to take him to Pondicherry.

Perhaps it was the Curse of the
Dodo or perhaps it was just bad
luck, but while he was waiting, Le Gentil learned that
war had broken out
between the French and the British, making a trip to British India very
difficult for a Frenchman.

Then the monsoon season started, meaning that even if he could find a
ship, it would have to take a much longer route to India than initially
planned and that it would be very difficult to make the journey before
the transit occurred.

Then, he caught dysentery for the first time.

Finally, after months of waiting, Le Gentil (barely recovered from his
illness) left Mauritius for India in February of 1761. Though time
appeared to be running out, the captain of the ship he was on promised
he would be there to observe the transit in June. About halfway to
India, the winds switched directions and the ship was forced to turn
back to Mauritius.

Le Gentil dutifully observed the transit of Venus in 1761 from a rocky
ship in the middle of the Indian Ocean. The data were useless and he
never attempted any analysis.

Although he missed the first transit, these things come in pairs
separated by eight years. There was still another chance. And with all
this time to prepare, there was no way he was going to miss the second one.

In fact, there was a bit too much time. But as a world-traveling 18th
century man of science, Le Gentil had plenty of other interests to fill
his days. He was particularly interested in surveying the region around Madagascar.

So he made a really nice map of Madagascar. And then he ate some bad
kind of some kind of animal and came down with a terrible sickness. He
describes this illness and its “cure” in his journals:

This sickness was a sort of violent stroke, of which several very
copious blood-lettings made immediately on my arm and my foot, and
emetic administered twelve hours afterwards, rid me of it quite
quickly. But there remained for seven or eight days in my optic nerve
a singular impression from this sickness; it was to see two objects in
the place of one, beside each other; this illusion disappeared little
by little as I regained my strength…

After recovering from both his sickness and the treatment, Le Gentil
decided to begin his preparations for the 1769 transit of Venus. He
calculated that either Manila or the Mariana Islands would be the ideal
spot to observe. The Sun would be relatively high in the sky at both
places when Venus passed by, meaning that the view would be through less
atmosphere with a reduced chance of clouds passing through the line of
sight. Le Gentil packed up his stuff and headed off to Manila, where he
could catch another ship to get to the Mariana Islands. Arriving in
Manila in 1766, the astronomer found himself exhausted from months of
sickness and sea-voyage. So, when he was offered passage on a ship
heading to the Mariana Islands, he quickly declined. That he chose not
to depart Manila at that time was perhaps his one stroke of good luck in
the entire journey. The ship sunk. Writing in his journal, Le Gentil
appears to have developed that particular sense of humor that generally
accompanies constant disappointment:

It is true that only three or four people were drowned, those who
were the most eager to save themselves, which is what almost always
happens in shipwrecks. I cannot answer that I would not have
increased the number of persons eager to save themselves.

In any case, Le Gentil was in Manila with plenty of time to prepare for
the next transit. Unfortunately, the astronomer may have over-prepared.
Having arrived three years before the event, he now had three years to
worry and second-guess his decision. It didn’t help that the Spanish
governor of Manila was kind of a crazy person. Not wanting to miss the
observation of a lifetime owing to the whims of mildly insane strong
man, Le Gentil packed up his stuff and headed to Pondicherry. Finally in
Pondicherry, Le Gentil worked tirelessly to construct his observatory
and make plenty of astronomical observations in preparation for the
event. He had state of the art equipment and had fully calibrated and
double checked everything. It was now nine years since his journey began
and only a few days until the transit was scheduled to occur at sunrise
on June 4th. The entire month of May was beautiful weather and pristine
observing conditions, as were the first few days of June. Le Gentil
likely went to bed on the 3rd of June fully confident that the next
morning would be no different. He woke up early in the morning to begin
preparations for his sunrise observations only to find clouds on the
horizon. The clouds remained, obscuring the sun, all through the
duration of the transit. A few hours after the end of the transit, the
sun broke through the clouds and remained visible for the rest of the
day. Le Gentil had missed his second transit in Pondicherry. He sums it
up in his journal:

That is the fate which often awaits astronomers. I had gone more
than ten thousand leagues; it seemed that I had crossed such a great
expanse of seas, exiling myself from my native land, only to be the
spectator of a fatal cloud which came to place itself before the sun
at the precise moment of my observation, to carry off from me the
fruits of my pains and of my fatigues

In Manila, the Sun rose in perfectly clear skies. Distraught, Le Gentil
remained in bed for some weeks afterward. He soon caught a fever and
missed the ship that was supposed to take him home. He recovered, but
then came down with dysentery again. Barely recovered from his various
illnesses, he managed to get a ride back to Mauritius. He caught a ship
leaving the island in November of 1770. The ship was struck by a
hurricane and almost completely destroyed. It managed to limp back to
Mauritius. The second attempt proved more successful and Le Gentil
finally “set foot on France at nine o’clock in the morning, after eleven
years, six months and thirteen days of absence.” Though he had finally
made it home, he was not out of the woods quite yet. In his absence, Le
Gentil’s heirs had tried to declare him dead to gain their inheritance,
his accountant had mishandled (and lost) a large chunk of his holdings,
and the Academy of Sciences, which had sent him on his 11 year mission,
had given his seat to someone else. It was not quite the welcome home he
had hoped for. Despite his seemingly never-ending misfortune, things did
turn around for Le Gentil. He married, had a daughter, and was
reinstated into the Academy of Sciences. Presumably, he lived out the
rest of his days in relative happiness. Le Gentil died in 1792. Keeping
true to his style, this man who missed two of the most important
astronomical events of his time fortunately managed to also miss the
most important (and violent) political
event of his time.

References:

I have mainly used a very nice series of historical papers of Le
Gentil’s misadventures with the transit of Venus written by Helen Sawyer
Hogg. The papers were originally published in the Journal of the Royal
Astronomical Society of Canada and can be accessed through NASA’s ADS
(Part 1,
Part2,
Part 3,
Part 4).

More Transit of Venus:

If you want to see the Transit of Venus without having to go on an
eleven year voyage (or even leaving your room), check out the NASAlive-feed from Mauna Kea.